As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for such mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and high discharge voltage, into which much research has been carried out and which is now commercially and widely used.
Depending upon the shape of a battery case, a secondary battery may be classified as a cylindrical battery having an electrode assembly mounted in a cylindrical metal container, a prismatic battery having an electrode assembly mounted in a prismatic metal container, or a pouch-shaped battery having an electrode assembly mounted in a pouch-shaped case formed of an aluminum laminate sheet.
Also, the electrode assembly mounted in the battery case is a power generating element, having a cathode/separator/anode stack structure, which can be charged and discharged. The electrode assembly may be classified as a jelly roll type electrode assembly configured to have a structure in which a long sheet type cathode and a long sheet type anode, to which active materials are applied, are wound in a state in which a separator is disposed between the cathode and the anode, a stacked type electrode assembly configured to have a structure in which a plurality of cathodes having a predetermined size and a plurality of anodes having a predetermined size are sequentially stacked in a state in which separators are disposed respectively between the cathodes and the anodes, or a stacked/folded type electrode assembly configured to have a structure in which a predetermined number of cathodes and a predetermined number of anodes are sequentially stacked in a state in which separators are disposed respectively between the cathodes and the anodes to constitute a unit cell, such as a bi-cell or a full cell, and then unit cells are wound using a separation film. The jelly roll type electrode assembly has advantages in that the jelly roll type electrode assembly is easy to manufacture and has high energy density per unit mass.
Meanwhile, a conventional mandrel used to manufacture an electrode assembly is formed in a hexagonal shape 92 or an angled oval shape in vertical section as shown in FIG. 1. Also, the entirety of the conventional mandrel is made of a single metal. The conventional mandrel has the following problems.
First, a Y-axis velocity change is greater than an X-axis velocity change, i.e. change of linear velocity at which a sheet type stack, such as an electrode, is wound on a mandrel 90 during rotation of the mandrel 90.
Also, force is concentrated upon a folded portion of the sheet type stack contacting an end 96 of a major axis of the mandrel 90, which has the maximum radius of gyration during rotation of the mandrel 90, with the result that acceleration when the sheet type stack comes into contact with the mandrel is increased at the folded portion of the sheet type stack, whereby the amount of impact applied to the folded portion of the sheet type stack is increased.
Furthermore, the linear velocity change at the folded portion of the sheet type stack and the amount of impact applied to the folded portion of the sheet type stack increase residual stress of the sheet type stack. When stress generated at a subsequent process, i.e. a process of compressing the jelly roll electrode assembly, is further applied to the folded portion of the sheet type stack, therefore, the folded portion of the sheet type stack is broken, whereby safety of a battery is lowered.
Consequently, the necessity of a technology for fundamentally solving the above-mentioned problem is very high.